1) Field of the Invention
The invention relates to an inductive position sensor for determining the position of a movable element, and particularly for determining the rotation angle of a movable element. However, the invention can also be used in a linear position sensor.
2) Description of Related Art
In a large variety of technical fields and for the most different uses, it may be required to detect, with the aid of measurement technology, the position of movable elements or parts of a components. An example for the use of an inductive rotary position sensor, taken from the field of automotive technology, is the detection of the position of the accelerator, the position of the throttle valve, the filling level of the gas tank, or the vehicle height level (i.e. degree of compression of vehicle suspension). The advantage of inductive sensor systems resides in the contactless capturing of a path position or rotary position.
The invention relates particularly to position sensors of the type schematically exemplified in FIG. 1 and described e.g. in WO-A-2004/072653, WO-A-2003/067181 and WO-A-2007/068765. FIG. 1, which will be described hereunder, illustrates a one-channel inductive position sensor for use as a rotary position sensor.
Said sensor 10 comprises two transmitter units 12,14 in the form of transmitter coils, each of them generating an electromagnetic alternating field with a position-dependent amplitude. Said two transmitter units 12,14 are controlled by a control unit 16, as will still be described further below.
Sensor 10 is further provided with a movable element 18 which in the present example is formed as rotary disk or another type of rotary element. Said movable element comprises an oscillating circuit 20 formed of an inductance 22 and a capacitance 24. Said element 18 or said oscillating circuit 20 will rotate within a total electromagnetic alternating field generated by the overlap of the two electromagnetic alternating fields of the two transmitter units 12,14. Depending on the respective rotational position, said oscillating circuit 20 will produce an electromagnetic alternating field of the same frequency as the alternating fields of the two transmitter units 12,14, wherein the alternating field emitted by oscillating circuit 20 is shifted in phase relative to the two other alternating fields. The degree of said phase shift is a measure of the present rotational position of element 18. The signal of oscillating circuit 20 will be received by a receiving unit 26 formed as a receiver coil and, within an analysis unit 28, said received signal will be processed and, particularly, there will be determined the phase position of said signal relative to the signals fed to the transmitter units 12,14.
The transmitter coils and respectively transmitter units 12,14 will modulate low-frequency oscillations of the same frequency (e.g. 4 KHz) onto a high-frequency (e.g. 4 MHz) carrier signal which is identical for both transmitter coils. The modulated oscillations of the two transmitter coils are phase-shifted by 90° relative to each other. Both transmission signals will energize the LC oscillating circuit 20. The strength of said excitation is proportionate to the coupled inductivity between the respective transmitter coils and the oscillating-circuit coil (inductance 22). Depending on the respective position of said movable element 18, each transmission signal will be coupled with a different strength into oscillating circuit 20. Within oscillating circuit 20, a modulated oscillation will be generated which has the same frequency as the transmitted modulation signal. The modulated oscillation of oscillating circuit 20 will have a phase shift, relative to the transmitted modulation, which is dependent on the amplitude ratio of the modulation signals—coupled into oscillating circuit 20—of the transmitter coils. The signal generated within oscillating circuit 20 will be forwarded to the receiver coil (receiver unit 26), as already mentioned above.
The mathematical approach forming the basis of the above measuring principle can be explained as follows. When adding to each other two sinusoidal oscillations of the same frequency which are phase-shifted by 90° relative to each other, a sinusoidal oscillation of the same frequency will be generated. The phase shift of the thus generated oscillation is a function of the amplitude ratio between the two added oscillations.
For many uses, an inductive position sensor with two or more channels is required. Then, said arrangement shown in FIG. 1, comprising the two transmitter units 12,14 with evaluation unit 16, the oscillating circuit 20 and the receiver unit 16 with evaluation unit 28 in the form of two subsystems, is provided twice and or more times. It is, however, not absolutely required that a separation exists between the subsystems; in the normal case, both channels are coupled into each other. A two-channel inductive sensor is shown, e.g., in FIG. 2 and is described, e.g., in US-A-2002/0179339 as well as in WO-A-2007/068765.
The known inductive sensors of the above mentioned design have been basically found useful in practice. However, for some uses, the current consumption of such sensors is occasionally too high. Further, since no sine or cosine signals are used at the input side, which is of advantage for an effective use of the system, a quite massive post-processing expenditure in the form of filtration processes and the like will be necessitated at the output side, which not desirable either and will increase the space requirement on an ASIC.
Known from US-A-2005/0030010 is an inductive position sensor of the type mentioned and described above, which is operated using a PWM signal as a modulation signal. Also this sensor requires an increased expenditure for signal post-processing, thus rendering the overall arrangement more complicated. For the filtration of the modulation signal so as to obtain the sinusoidal or cosinusoidal shapes, a low-pass filter with relatively low limiting frequency will be required, which will entail the need for additional circuit components and thus cause an increased space requirement in the ASIC.
A method for generating the transmission signals that is advantageous for the monolithic integration resides in the generation of square-wave signals whose shape, with filtration over time, corresponds to the desired signal shape of the modulated transmission signal. The use of square-wave signals allows for high efficiency, which is achieved by avoidance of losses in the integrated circuit. Generating square-wave signals further makes it possible to achieve a particularly high linearity of the sensor signal because of the high relative accuracy with which the square wave signals can be generated in integrated circuits. Square-wave signals can be used both for generating the carrier signal and for generating the modulation signal.
A method for generating the modulation signal that is of special advantage for integration resides in using either a pulse-density-modulated sequence of carrier-frequency square wave pulses whose shape, with filtration over time, corresponds to the desired modulation signal, or a sequence of carrier-frequency square wave pulses whose polarity can be reversed after each half period of the modulation frequency. Both methods eliminate the necessity of an analogous multiplication for generating the modulated signal and thus allow for a particularly high linearity of the sensor signal. The filtration of the generated square wave signals which is required for evaluation is advantageously performed partially in the resonance circuit of the movable element and partially in the reception path of the integrated circuit.
As described above, for a large variety of uses, inductive position sensors are given a design with multiple channels. In doing so, the subsystems are often provided in close spatial proximity. Due to the spatial closeness of the coils of different subsystems and in consideration of the further boundary conditions in the designing of the coils (e.g. suitable strength of the coupling factors between the transmitter and the oscillating circuit and between the oscillating circuit and the receiver, highest possible similarity between the inductivities and between the resistances in both transmitter coils of a subsystem, correct angle-dependency of the coupling factors, suppression of the far field of all individual coils, etc.), it will not be generally possible to avoid a mutual coupling of the coils of different subsystems.
This inductive coupling of the coils of the two subsystems (e.g. the oscillating circuit of channel 1 to the receiver of channel 2, or the oscillating circuit of channel 2 to the receiver of channel 1) has the consequence that, to the receiver of one subsystem of a multi-channel sensor, signals of the other subsystem will be supplied. If these coupled-in interference signals of the foreign subsystem are not sufficiently suppressed in the receiver, they have a negative influence on the measured sensor signal. Thus, the described influences from neighboring channels will particularly cause increased noise (stochastic fluctuations) of the measured position signal.